I have a non-electronic key that I carry with me on the dive.
(Separate from my key ring with the electronic fob on it...)
While I only dive dry, with the spare key bolt-snapped off to a loop inside my thigh pocket, the key would also work with a wetsuit - the key doesn't care if it gets wet. (Like Spectrum, I also leave a water bottle outside the van to rinse off the key after the dive...)
Even if you have a key with a chip in it, most dealers will - for a small fee - be able to provide you with a 'chip-less key' that fits into your wallet. That would be the one to take with you on the dive.
Before I went this route I just tucked my key ring either up on top of the front tire inside the wheel well, or else behind the front tire up on the frame of my van.
But even though its quite true that someone who wants to break in might easily do so even without the key, I see no reason to make it easier for them - and they know the relatively easy places to look.
Making it more difficult for them serves some deterrent value...perhaps they'll go look for a vehicle thats easier to break into.

(Parking intelligently may also play some role in determining who gets hit...)
Best,
Doc